1. Field of the Invention
The present invention relates to an information processing device connected to a local area network that is connected to an outer network through a router. More specifically, the present invention relates to an information processing device capable of causing a router to open a new communication port in a case where a terminal connected to an outer network needs an access and also capable of accepting an access from an outer terminal. The term “communication port” in the present specification refers to a logical communication channel that is opened by a device (typically, a device referred to as a “router”) connecting two networks.
2. Description of the Related Art
An information processing device connected to a local area network (hereinafter called “LAN”) needs to be protected against any unauthorized access by a terminal (hereinafter called “outer terminal”) connected to an outer network. An information processing device connected to a LAN may hereinafter be called an “inner terminal” in order to distinguish it from the outer terminal. In order to prevent any unauthorized access to the inner terminal, a router connecting the LAN and the outer network restricts an active access by the outer terminal to the inner terminal. In the present specification, “an active access by the outer terminal to the inner terminal” refers to an access where a communication is initiated by the outer terminal. Specifically, the router restricts an active access by the outer terminal by restricting a communication port (hereinafter often simply called “port”) that opens to an outer network to which the outer terminal connects.
In a case where the inner terminal connected to the LAN communicates with the outer terminal, an access from the outer terminal to the inner terminal is usually permitted after the inner terminal initiates the communication. The term “initiate” is synonymous to “actively access” in the present specification. The router permits an access from the outer terminal to the inner terminal in a case where the access by the outer terminal is a response for the access by the inner terminal to the outer terminal. More specifically, when the router receives an access from the inner terminal to the outer terminal, the router opens a new port for receiving a response from the outer terminal in regards to the aforesaid access by the inner terminal, and transfers the access by the inner terminal to the outer terminal. When transferring the access by the inner terminal to the outer terminal, the router informs the outer terminal a port identifier of the opened new port. The outer terminal then in response transmits data for accessing the inner terminal with the port identifier designated. When the router receives the access by the outer terminal with the designated port identifier, the router transfers the access to the inner terminal. In TCP/IP, this function of the router is realized by a port translation process referred to as an NAPT (Network Address Port Translation) or an IP masquerade, for example. For example, a typical port translation process is executed by a router in a case where an inner terminal accesses a POP server (POP server is a kind of the outer terminal) and acquires an electronic mail (hereinafter abbreviated as “mail”) addressed to it from the POP server.